The overall objective of the proposed study is to delineate the detailed innervation patterns of the extraocular muscles. We plan to integrate information concerning motoneuron organization, nerve fascicle arrangement and muscle innervation patterns in the IIIrd, IVth and VIth nerves of the cat. We will use nerve dissections and horseradish peroxidase (HRP) labeling of specific nerve branches in conjunction with serial histological and ultrastructural studies of the ocular motor nuclei, nerve trunks and extraocular muscles. Specific whole muscle nerves and intramuscular branches of these nerves will be anatomically described in initial studies. Specific branches will then be surgically isolated, transected and labeled with HRP in subsequent experiments. For example, the retractor bulbi muscle branch of the VIth nerve or one of the initially described intramuscular nerve branches in the lateral rectus muscle will be labeled. The histochemically labeled axons of this nerve branch will be traced through sections of nerve trunk, brainstem and, where possible, into the innervated muscle. Anterograde labeling in the distal nerve branch stump will allow muscle fascicle and muscle fiber innervation patterns to be further categorized. The retrograde labeling from the proximal end of the nerve branch will allow nerve fascicle and motoneuron pool organization to be assessed. We propose that these studies will enable us to see whether motoneurons within the nuclei are organized in register with the arrangement of axons in their nerves and/or with the nerve endings in their muscles of innervation. This nucleus-nerve-muscle organization data is necessary for determining the degree of structural order available to the nervous system for effecting coordinated muscle activation. This information, finally, may be pertinent to ophthalmic surgeons who routinely effect changes in the eye movement control system at the periphery.